Wollastonite particles have long been employed as filler material in the manufacture of thermoplastic molding compositions (Modern Plastics Encyclopedia 1977-1978, McGraw-Hill Inc., New York, N.Y.), p. 183 ff. By use of these particles, which are inexpensive, compounders of thermoplastic molding compositions have found it possible to effect a major decrease in the cost of their molding compositions with no more than a minor sacrifice in the principal physical properties of the molded pieces obtained. However, a demand has arisen for particles which would cause even less of a decrease in physical properties, and the present invention is directed to this problem.
Wollastonite is a native mineral (a calcium metasilicate of the pyroxane group), and is hydrophilic. When crushed it provides particles which are acicular in shape and which are heat and abrasion resistant. The particles which are between 1.mu. and 1000.mu. in length (and preferably between 1.mu. and 40.mu. in length) are the fraction which is used as filler in the compounding of thermoplastic molding compositions.
The discovery has now been made that the surface of wollastonite filler particles carries reactive sites and that the affinity of these particles for aromatic polycarbonate resins is improved when the particles have been treated with an aryl epoxy compound as is more particularly described below, so as to provide the particles with a surface which is passive.
The further discovery has been made that the coatings of the thus-treated particles are thermostable in that they do not appear to decompose in the temperature range (400.degree.-550.degree. F.) at which compounding of aromatic polycarbonate molding compositions customarily takes place.
The nature of the aforesaid reactive sites, the manner in which the aryl epoxy compounds inactivate these sites or passivate the surface of the particles, and the cause of the improved affinity of the treated particles for aromatic polycarbonate polymers have not been ascertained, and applicants do not wish to be bound by any theory.
In its broadest scope, therefore the present invention provides aromatic polycarbonates in combination with novel wollastonite filler particles, said particles carrying in combined state a small but effective amount, in the range of 0.01% to 10%, of the weight of the particles, of an aryl epoxy compound as agent improving the modulus and other properties of the thermoplastic composition.
The invention further provides a thermoplastic molding composition wherein an aromatic polycarbonate is the principal structural component and the composition has a uniformly distributed content, in the range of 0.1% to 400% of the weight of said polymer, of the above-described treated wollastonite particles.
For convenience, hereinafter the aryl epoxy compound on the particles will be described as being "combined" therewith, and it will be understood that this term will describe the manner of combination whether it be by reaction, by hydrogen bonding, by absorption, or otherwise.
The aryl epoxy compounds with which the wollastonite particles are treated, are those aryl epoxy compounds, monomeric or polymeric, which can be deposited on the particles in fluid state and which combine with the particles so as to render their surfaces passive or inert. Two or more epoxy groups should be present. In the case of polymers, the density of epoxy groups should be between 0.1 and 4 epoxy groups per functional unit.
Suitable polymers include those prepared from A-stage phenol-formaldehyde condensation products and epichlorohydrin, and the linear polymer prepared from p-vinylphenol and epichlorohydrin. The aryl ring may be phenyl, naphthalene or phenanthrene.
In practice the monomeric reaction product of epichlorohydrin with bisphenol-A having the theoretical formula: ##STR1## is preferred as this compound is readily prepared from inexpensive materials and has an epoxy density of 2. It is available, e.g., from Dow Chemical Co. under the tradename DER 332.
It is advantageous to employ the epoxy treating agent in moderate (10% to 20%) excess, to ensure that substantially all the surfaces of all the particles are rendered passive. The amount of excess needed for the purpose varies from sample to sample of wollastonite and with the epoxy density of the treating agent. The minimum amount of excess in any instance is therefore most conveniently found by making a series of laboratory trials. Extraction of the treated dry wollastonite with an inert solvent for the epoxy compound reveals that the amount of epoxy compound which is retained by the wollastonite is small, and is usually in the range of 0.01% to 3% of the dry weight of the particles. Evidently, then, the coating may be monomolecular or even less, and in any event is very thin.
Any method for applying the epoxy compound to the wollastonite particles can be utilized which causes uniform distribution of the agent as a fluid coating over the surfaces of the particles and which provides conditions under which the epoxy compound can combine with the active sites on the surfaces of the particles.
Thus one feasible method is to heat a batch of wollastonite filler particles to a temperature at which the treating agent is fluid and then to add the treating agent and the wollastonite to an extruder mixer of a conventional type which provides shear. Slow mixing is effected over a long period of time to ensure complete coating of the particles and passivation of the active sites thereon.
A more rapid method is to employ a volatile inert liquid as solvent and carrier for the treating agent. According to this method the treating agent is dissolved in a small amount of an appropriate solvent and the solution is added to the wollastonite batch which is to be treated, following which enough more solvent is added to provide a slurry of whatever consistency is desired. Preferably sufficient solvent is added so that there is a continuous liquid phase. The mixing may be and preferably is performed at room temperature. The solvent is then removed by use of vacuum so as to deposit the epoxy compound uniformly on the particles, after which the batch is heated to a reaction temperature (usually in the range of 75.degree.-200.degree. F.) and allowed to age until the epoxy compound has combined with active sites as aforesaid.
Following either method, any excess (uncombined) epoxy agent can be removed. This can be done by extracting the batch with an inert solvent, preferably at an elevated temperature to facilitate separation of the excess from the particles. In most instances, however, and especially when only a small excess of the treating agent is present, the removal step is unnecessary because the excess dissolves in the molten structural polymer or decomposes or volatilizes during the compounding step. In any event, allowing the unreacted excess epoxy compound to remain in the filler batch does not appear to have a detrimental effect on the strength and impact resistance of articles molded from the composition.
Aromatic polycarbonate polymers which are suitable for use as the structural component of the molding compositions of the present invention are well known and are described, for example, in The Encyclopedia of Polymer Science and Technology, Vol. 10, New York, 1969, pages 710-764.
Preferred are the polyaromatic polycarbonates represented by the polymer known as Lexan (manufactured by the General Electric Co., Pittsfield, Mass.) which is substantially composed of units derived from bisphenol-A and phosgene, and having the theoretical formula ##STR2## Also useful are the di- and tetrabromo and the di- and tetramethyl derivatives thereof.
Furthermore those skilled in the art can easily substitute polycarbonate blends with conventional, compatible resins, polycarbonate ester copolymers, polycarbonate siloxane copolymers, and the like. See the above-mentioned literature reference incorporated herein by reference.
The epoxy compounds and structural polymers which have been mentioned above can be employed in admixture with other respectively similar polymers which are now known or which may hereafter be devised. The epoxy compounds are described in The Encyclopedia of Polymer Science and Technology, Vol. 6, Interscience, New York, 1967, pages 209-271, incorporated herein by reference.
The compositions of the present invention may and usually do contain one or more of the auxiliary components which are customarily present in present-day commercial molding compositions. These components include pigments; ultraviolet ray absorbers; flame retardants of any suitable type including potassium diphenylsulfone-3-sulfonate and sodium 2,4,5-trichlorobenzenesulfonate; plasticizers; supplementary fillers; foaming agents; and fibrous strengtheners for example, glass and asbestos fibers.
The proportions in which these auxiliary components are employed, and their manner of compounding with the structural polymer or polymers are conventional and are not features of the invention.
After the compounding step the compositions of the present invention can be prepared as a homogeneous free-flowing particulate product by extruding the melt containing the aromatic polycarbonate, the filler and auxiliary agents in the form of strands, cooling the strands until solid, and comminuting the strands to a desired particle size. The resulting product can be fed directly into an injection or compression molding machine.